The invention relates to a movement measurement with reference to an inertial system. Specifically it relates to measurements in which the movement of an object is measured by using a laser or other radiation beams generated from a resonator with an output.
FIG. 1 is a schematic view showing conventional vibrometer using a laser beam, the extracting system, the coupling system and the receiver for measuring difference signal of the radiation beams (laser beams).
In FIG. 1, an optical vibrometer according to the U.S. Pat. No. 4,768,381 (Sep. 6, 1988) comprises a reflecting member 11 having a conical concave reflective surface 12 disposed on an object 3 to be measured, a light source 1 for generating light 4 to irradiate the reflective surface 12 of the reflecting member 11, a vibration measuring means 2, which comprises a beam splitter and a coupling system, disposed between the light source 1 and the reflective surface 12 of the reflecting member 11 for measuring the three dimensional vibration of the object 3 based on the light 4 generated by the light source 1 and the light 5 reflected from the reflective surface 12 of the reflecting member 11.
In the above-mentioned vibrometer, as in the all Doppler-effect based devices, however, there is not provided the autonomous measurement of an irregular movement of an object, when measurement means are fully disposed on the object.
This results in decreasing of both measurement accuracy and cost efficiency.
FIG. 2 is a schematic view showing an another conventional vibrometer with the polarizing elements on the optical fiber to measure an ultrasonic vibration of the substance.
In FIG. 2, an optical fiber ultrasonic sensor according to the U.S. Pat. No. 5,297,436 (Mar. 29, 1994) comprises a laser source 1 and a quarter wave plate 2 through that the output from the laser is directed to provide a circularly polarized beam.
The laser source can be a gas laser or laser diode providing a linearly polarized output.
The output from quarter wave plate 2 is directed to an input end 3 of a polarization maintaining fibre 4 of the substantially known type.
Fiber 4 includes a substantially straight sensing portion 5 that is exposed to the incident ultrasonic wave to be measured schematically shown at 6. Sensing portion 5 is mounted by the suitable known means illustrated at 7 so that the fast principal axis of the fiber is aligned with the propagation direction of the ultrasonic wave to achieve maximum induced phase difference.
Light from output end 8 of fiber 4 are directed to a beam splitter 9. One beam passes through the linear polarizer 10 with its principal axis rotationally displaced at 45.degree. to the principal axes of the fiber and the intensity of the transmitted beam is detected by the photodetector 11. The intensity of the other beam from the beam splitter 9 is detected by the photodetector 12. Photodetectors 11 and 12 can be of any suitable type for example a PIN photodiode or an avalanche photodiode followed by an electronic amplifier.
The output signals from each of photodetectors 11 and 12 are directed to a signal processor 13 which in turn generates a suitable driving signal for a display unit 14. The display unit can, for example, be an oscilloscope, spectrum analyzer or computer.
However, the above-mentioned optical fiber ultrasonic sensor comprises added sensitive elements and materials, and there is not provided an autonomous measurement.
As mentioned above, in both of conventional devices there are moving and motionless parts, added elements and materials. This results in decreasing of both measurement accuracy and cost efficiency.